1. Field of the Invention
The present invention relates to a suspension arm unit having: a base end part vertically swingably supported on a vehicle body side about an axis extending in a front-rear direction of a vehicle; and a distal end part to which a knuckle arm unit of a wheel is attached.
2. Description of the Related Art
One example of this type of suspension arm is disclosed in Japanese Unexamined Patent Application Publication No. 2001-121933 (paragraphs 0020-0023, FIG. 1). In the suspension arm described in this patent document, a pair of fork-shaped members each extending in two directions from a single distal end part to two different base end parts are butted in a vertical direction, and side faces of two members are welded together to unify two members into a single piece. Two base end parts of the thus obtained arm are apart from each other in a front-rear direction of the vehicle, and vertically swingably supported on a vehicle body side. Each fork-shaped member has a U-shaped cross section and while openings of U shapes are butted each other in the vertical direction, the fork-shaped members are welded together except the distal end parts. Therefore, the arm except the distal end part has a hollow structure with a rectangular cross section. On the other hand, the distal end part has a plate structure which does not form a hollow structure even when two fork-shaped members are coupled in the vertical direction, so that a ball joint for connecting and supporting a knuckle arm is fixed simply with screws, without using a conventional mount member.
In the above-described conventional suspension arm, each arm member before welding has the U-shape as a cross sectional view and a fork-shape as a planar view having curved branches extending from the single distal end part to the two base end parts. In addition, the base end part and the distal end part should have totally different specific cross-sectional shapes. Therefore, in order to manufacture these arm members, special press molds each having a special curve shape are required, leading to a problem that a production cost is likely to become high. In addition, even when a slight design change is applied to, for example, a branching angle of the arm member, a new special press mold should be prepared.
Further, in a case where the distal end part having the closed rectangular cross section to which a knuckle can be attached is fabricated from a single steel plate using bending processing, the bending processing should be applied to at least three parts of the plate. Therefore, special press jigs and advanced press techniques are required, leading to an increase in the number of processing steps, and again the production cost is likely to become high.